Light emitting diodes (LEDs) are increasingly used for a variety of lighting applications—and in particular are increasingly employed within automotive front lighting. LEDs offer a number of advantages over traditional (incandescent or fluorescent) light sources, including long lifetime, high lumen efficiency, low operating voltage and fast modulation of lumen output.
LEDs also open up additional functionalities within automotive lighting. In particular, pluralities of LEDs employed together within a single lighting unit, offer the possibility for adaptive beam shaping. An array of LEDs, for example, having individual or group addressability may be employed to selectively generate beams of differing shapes, angles and profiles. Used, for example, in combination with external beam-shaping optics, highly directional, high contrast front beams may be generated with the additional capability to be dynamically adapted in real time to realise different shapes, directionalities or angular widths.
Adaptive beam shaping of this sort requires an array of LED modules which can be activated or deactivated individually, or within small subgroups—though individual addressability is preferable in virtue of the greater flexibility which it affords. Typically individual addressability of LEDs within an array is achieved through providing wiring tracks which run along the intermediary spaces between array rows and/or columns, in order to reach and provide individual currents to every LED across the expanse of the array
However, to achieve maximal performance in such applications, it is preferable that the light intensity is maximised, requiring, for a given number of light sources of known output power, that the total area over which light is emitted be minimised. For an array of LED modules, minimised light-emitting area requires the spacing between adjacent modules to be as small as possible.
Wiring tracks running along (i.e. parallel and coincident with) spaces between array rows and/or columns, as utilised by state of the art devices to enable individual addressability, consume valuable space, thereby limiting the achievable density of the array, and hence the light intensity of the device.
One solution to this problem has been to instead route some or all of the individual connecting tracks, not between the LED modules, but below them, as a separate layer incorporated within the substrate itself (see for example FIG. 4).
However, multi-layer wiring solutions of this sort carry the significant disadvantage of substantially limiting heat dissipation capacity within the device. Within a closely packed array of LEDs, heat cannot spread laterally (in an X-Y plane), due to the close proximity of neighbouring LEDs, and hence must be dissipated ‘vertically’, through the substrate layer, typically to a dedicated heat-sink layer below. This requires optimal thermal path between the LED modules and the heat sink. One or more wiring layers running through the substrate layer impede this thermal path, significantly diminishing the thermal conductivity between the LEDs and the heat-sink. Additionally such wiring layers also increase costs, since a multilayer-type substrate is required, and also risk compromising reliability, since vias are required for connecting to internal layers.
Desired, therefore, would be lighting unit comprising a closely arranged array of LEDs, having a wiring scheme which facilitates individual addressability of component LEDs, but while avoiding (or at least greatly reducing) the need for wiring tracks which run either below, or along the intermediary spaces between, LEDs thereby maximising heat dissipation rate to the heat sink, while also minimising total incurred area footprint.